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Angles in the Infield
Name ________________________
The ancient Greeks loved Geometry. Twenty-three hundred years ago, Euclid wrote the first book on
Geometry, a book that is amazingly similar to the Geometry texts of today. The word Geometry has a
prefix "Geo" which means earth. The base of the word, "metry," means measure. So the Greek word,
Geometry, means_______________________.
1) In this unit you will be introduced to the language of Geometry. The Greeks believed that
mathematics is the language of the universe. Points, lines and planes are the three building blocks of
Geometry. Rule #1: Points are always named with capital letters.
2) Segments are made from an infinite amount of points, but a segment is always named by the two
endpoints. This is segment AB or segment BA.
This is line AB or line BA:
This is Ray BX or Ray BA
Rays are named with two capital letters, starting at the only endpoint. For example, if I told you to
draw ray AB, it would start at point A and go to the left through point B.
3) How many endpoints does a ray have? ________
4) Why is it impossible to measure the length of a ray? _______________________________
5) In this handout you will study angles.
An angle is just two rays with the same endpoint.
They are named by the point w here the rays
meet, that point is called the vertex.
What is the name of the vertex of our angle? _________
So this is called ANGLE C, but sometimes in order to avoid confusion, we have to name the angle
using all three letters. When you do, just make sure the vertex is the middle letter in the name.
A second way to name this angle is  ACB (read: angle ACB). What's a third way? _______
6) That angle could not be named  ABC, because that would be a completely different angle. Draw an
 ABC on the picture above, don't forget to put arrows on the rays.
7) Here are three angles. Can you see them?
Name the smallest ________ (Use 3 letters for all.)
Name the largest ________
Name the middle-sized one _______
8) Explain why we can't tell you to just look at D?
We have to use three letters to name any one of these angles because __________________
9) Ray DE is a side of both PDE and also EDA. When two angles share a ray they are called
adjacent angles. Explain what the word adjacent means.
Angles may not be measured as you think.
Which angle is the biggest ? ________
I hope you picked C. It is not the length of the rays that make the angle bigger. Please explain
what you think determines the measure, or size, of an angle.
We measure angles by how far they have "opened." As you look at the picture above you can see  C is
bigger than  B because it has opened up more. So angles aren't measured in inches or centimeters,
instead we measure angles in DEGREES.
Here is angle with a measure of about one degree, or 1°. It has barely started to open.
Pretty skinny, huh? The tool we use to measure them is a protractor. (Hey, who said thermometer?)
There are three easy steps to
measuring with a protractor:
First Put the hole (or ) of the
protractor on the vertex .
(Line it up as with "bomber sights.")
Second Third -
Line up one of the rays with the zero mark.
Just count around the protractor until you get the other ray.
Measure these angles to the nearest degree. Take pride in your work and try to be very precise.
Remember, bomber sights, 0 on a ray, and count.
12) ______
a) ______ (label?)
b) Measure the angle made by any one corner of this paper. _______ __9-2 page 3
in the Infield
Name ________________________
Angles are classified by their size:
if they are less than 90° they are called acute,
90° angles are right angles,
and angles from 90° to 180° are called obtuse.
(Here's a trick to help you remember which are which. The little ones are cute, while the big ones look to
be obese.)
Measure these angles. Classify as acute, right, or obtuse. You may need to extend some sides so you
can line it up with your protractor. (Remember: Bomber sights, zero, and count.)
14) ______, _________(Extend the bottom ray.)
17) ACR_____, _____________
RCE _____, _____________
ACE _____, _____________
I went sucker fishing on the last question. Did I catch you? ________ We can't call one particular
angle –C, because there are really _________ angles which could claim that name.
Now you will get to draw angles of a specific size.
First Draw a ray.
Second Put the hole ("bomber sights") of the protractor on the end of the ray.
Third Line up zero, count to the degrees you need, and make a small mark.
Fourth Draw a ray from the vertex to the mark
Draw a 30 degree angle; it is started for you. Remember: ray, sights, zero, and mark.
Draw a 72 degree angle "below" this ray.
Draw a 135 degree angle.
Draw a 90 degree angle, start with a new ray.
Draw a 180 degree angle, which is called a straight angle. Just make sure both rays show.
What kind of angles do you see in the infield of a baseball diamond? ______________
Do you have any questions on drawing angles?